1. Field of the Disclosure
The present disclosure relates to a wavelength selection switch capable of branching or coupling light components with different wavelengths in optical wavelength multiplex communication.
2. Discussion of the Background Art
With the spreading of optical wavelength multiplex communication, a wavelength selection switch that multiplexes or demultiplexes an optical signal for each wavelength has been used as a key device in optical communication.
FIG. 1 is a diagram schematically illustrating the structure of a wavelength selection switch according to the related art. Input/output ports 101 indicate all input/output ports (in FIGS. 1 and 2, input/output ports 101a to 101e) in an input/output end 100. And waveguides 14 indicate all waveguides (in FIGS. 1 and 2, waveguides 14a to 14e) in a fiber array 140.
A wavelength selection switch 200 shown in FIG. 1 includes: a lens array 102 that converts light output from the input/output ports 101 of the input/output end 100 arranged at a focal position into parallel light; a first lens 103 with a high numerical aperture that focuses light emitted from the lens array 102; a second lens 104 that is provided so as to have the same focal position as the first lens 103; a spectral element 105 that reflects light emitted from the second lens 104 at different angles for each wavelength; and a mirror array 106 that is provided at the focal position of the second lens 104 and reflects light traveling from the spectral element 105 through the second lens 104 to the second lens 104 at an arbitrary angle (for example, see Patent Documents 1 to 3).
In this way, the light components reflected at an arbitrary angle by the mirror array 106 are focused on different input/output ports for each wavelength according to the inclination angle of each micromirror in the mirror array 106. As such, the first lens 103 has a function of changing the angle of light that has been reflected from the spectral element 105 again and then passed through the second lens 104 among the light components reflected by the mirror array 106 and giving an offset to the optical axis of light from the input/output ports 101.
FIG. 2 is a diagram schematically illustrating the structure of a wavelength selection switch 200′ having a transmissive spectral element 105′ instead of the reflective spectral element 105 shown in FIG. 1. FIG. 2(a) shows the wavelength selection switch 200′ in the x-z plane and FIG. 2(b) shows the wavelength selection switch 200′ in the y-z plane. However, in FIG. 2, for simplicity of illustration, both the incident angle and the diffraction angle of the transmissive spectral element 105 are approximately zero, but both the incident angle and the diffraction angle are actually close to 45 degrees. Components having the same functions as those of the wavelength selection switch 200 shown in FIG. 1 are denoted by the same reference numerals. A third lens 104′ shown in FIG. 2 has the same function as the second lens 104 shown in FIG. 1 with respect to light from the spectral element 105′ to the mirror array 106.
In the case of a wavelength selection optical switch having N inputs and one output (Add type), one of the input/output ports of the wavelength selection switch 200′ shown in FIG. 2 may be an output port and the other input/output ports may be input ports. In the following description, the input/output ports are referred to as an output port 101c, an input port 101a, an input port 101b, an input port 101d, and an input port 101e. 
Wavelength-multiplexed optical signals are emitted as diverging light from the input ports 101 through the waveguides 14 in the fiber array 140. For example, input light (two-dot chain line) emitted as diverging light from the input port 101b is incident on the lens array 102, is converted into parallel light, and is then incident on the first lens 103. The input light incident on the first lens 103 is converted into converging light, is focused on a first imaging point A, is changed to diverging light again, and is then incident on the second lens 104. The diverging light is converted into parallel light again and is then incident on the spectral element 105. The input light incident on the spectral element 105 is demultiplexed into light components for each wavelength. The demultiplexed light is incident on the third lens 104′, is changed to converging light, and is then focused on the mirror array 106 for each wavelength. For example, a micromirror 106c is inclined at an angle θm required to make output light incident on the output port 101c, and output light (solid line) from the micromirror is incident as diverging light on the third lens 104′. The output light is converted into parallel light by the third lens 104′, passes through the spectral element 105, and incident on the second lens 104. The light incident on the second lens 104 is converted into converging light and is then focused on the first imaging position A. The output light focused on the first imaging position A is incident as diverging light on the first lens 103, is converted into parallel light, and is then incident on the lens array 102. The light incident on the lens array 102 is converted into converging light, is coupled to the output port 101c, and is propagated through the fiber array 101.
The wavelength selection switch according to the related art includes two confocal optical systems. That is, the wavelength selection switch includes a confocal optical system I including the lens array 102 and the first lens 103 and a confocal optical system II provided in the rear stage of the confocal optical system I. In FIG. 1, the confocal optical system II includes the second lens 104 and the spectral element 105. In FIG. 2, the confocal optical system II includes the second lens, the spectral element 105, and the third lens 104′. When the focal length of the lens array 102 is fo and the focal length of the first lens 103 is f1, the image magnification M1 of the confocal optical system I is f1/fo. The image magnification is the absolute value of a lateral magnification. When the mode field diameter of the fiber is ωo, a beam spot size ω1 at the first imaging position A is represented by the following Expression 1:ω1=ωo·f1/fo.  [Expression 1]
Light passes through the same lens in FIG. 1, and light passes through two lenses having the same lens characteristics in FIG. 2. Therefore, the image magnification of the confocal optical system II is 1 and the beam spot size at the first imaging position A is equal to that on the mirror array 106. That is, a beam spot size ωm on the mirror array 106 is represented by the following Expression 1′:ωm=ωo·f1/fo.  [Expression 1′]
A beam size ωg on the spectral element 105 is represented by the following Expression 2 from Expression 1 and the following Gaussian beam equation:ωg=λ·f2/(π·ω1); and  [Gaussian beam equation]ωg=λ·f2·fo/(π·f1·ωo)  [Expression 2]
(where f2 indicates the focal length of each of the second lens 104 and the third lens 104′).